Feasibility of a low-energy X-ray free-electron laser

X-ray free-electron lasers based on self-amplified spontaneous emission promise users unprecedented X-radiation that is extremely bright, extremely short and transversely coherent. However, hard X-ray free-electron laser facilities under construction are all huge and expensive, consisting of high-energy linear accelerators and long undulators. The benefit of hard X-ray free-electron lasers may be limited to only a few regions in the world, unless it is possible to reduce the size. This paper discusses how small a hard X-ray free-electron laser facility can be. It is shown that a 1.5 A X-ray free-electron laser is achievable using electron energy down to 4.5 GeV or lower if we use the third-harmonic radiation, but at the expense of the transverse coherence.